"Personally I think this is really important, possibly more important than anything else we'll do this year, even with our O.W.L.s coming up. I, personally, am at a loss to see why the Ministry has foisted such a useless teacher on us at this critical period. Obviously, they are in denial about the return of You-Know-Who..."

During his second year, Ernie came to believe that Harry Potter was the Heir of Slytherin after he spoke Parseltongue in front of the Duelling Club. Harry was trying to stop the snake conjured by Draco Malfoy from attacking Justin, but Ernie perceived it differently, believing that Harry was in fact urging the snake to attack. He stepped in to protect his friend urging him to hide their common room. He went into discussion with the other Hufflepuffs regarding if Harry was the heir of Slytherin, considering the coincidences with the victims. He voiced that being a Parselmouth was the very symbol of a dark wizard and is probably the reason why Voldemort wanted to kill Harry; he didn't want to have another Dark Lord competing against him. All the while, Harry, who was attempting to explain to Justin what really happened, was eavesdropping on their conversation and attempted to convince Ernie that the altercation at the Duelling Club was not what it seemed. When it seemed that their argument was going nowhere, as Ernie cited that Harry hated his Muggle relatives, the latter left the library.

Soon enough, Ernie's fears worsened when Justin was petrified.He discussed the matter with their mutual friend Hannah, who was initially sceptical because she thought Harry was nice, but was eventually persuaded to Ernie's point of view.

However, Hermione Granger, one of Harry's best friends, was later petrified as well. This made Ernie realise that Harry could not be the Heir of Slytherin, as he knew Harry would never harm Hermione, and he gave Harry a public apology. He believed that Malfoy could be the culprit, but Harry gave an emotionless no, making Ernie and Hannah stare.[3]

In his third year, Ernie took many subjects similar to Hermione. He once told Ron Weasley that she had never missed a Muggle Studies lesson, despite having another class at exactly the same time that she also attended. This was due to Hermione's use of a Time-Turner to attend all her lessons.[4]

Ernie's fourth year held place to the Triwizard Tournament, which was hosted at Hogwarts that year, he supported fellow Hufflepuff Cedric Diggory when he qualified and again stopped speaking to Harry Potter, who also qualified, but underage, as he was one of many students who initially believed that Harry had entered himself into the tournament in a bid for further glory. He tried to convince friends in Hufflepuff to agree with him, but they needed not much convincing.

He also wore a Support Cedric Diggory/Potter stinks badge[5] After the tournament came to a close Cedric Diggory was murdered by Lord Voldemort, coming as a shock to all students and staff members. Cedric Diggory was a member of Hufflepuff and since Harry was with him when he died Ernie appears to have forgotten any ill wishes he thought of Harry and believed his claims to Voldemort returning whilst the Ministry didn't.

"I'm doing eight. Eight or nine. I'm getting an hour in before breakfast everyday. Eight's my average. I can do ten on a good weekend day. I did nine and a half on Monday. Not so good on Tuesday — only seven and a quarter. Then on Wednesday —"

In 1995, Ernie became a Hufflepuff Prefect, alongside Hannah Abbott. Unlike previous years, during which he had thought ill of Harry for certain acts he believed him to have committed, Ernie was one of the few Hogwarts students who believed Harry's story that Lord Voldemort had returned. He even went as far as to make an openly public declaration of support on Harry's behalf.[6]

Shortly after the beginning of term, Ernie joined Dumbledore's Army because of his belief in Harry and because of his dismay with Dolores Umbridge, a Ministry of Magic employee who became Defence Against the Dark Arts professor but refused to teach students any practical defence. Ernie believed Dumbledore's Army to be one of the most important things they would be doing that year—apart from O.W.L.s. In the run-up to his examinations, he would study between eight and ten hours a day. He developed the irritating habit of boasting about this fact, then inquiring about other students' study habits.

Later in the year Death Eaters invaded Hogwarts, Dumbledore's Army was called to battle and the Battle of the Astronomy Tower broke out, it's likely Ernie never received his D.A. signal as he had left the Hufflepuff Common Room to investigate a disturbance he had heard when he saw Draco Malfoy, Severus Snape, and Harry Potter running down a corridor. He asked Harry what was going on, but got no response. He would later discover that Albus Dumbledore had been killed that night by Snape, and then presumably attended his funeral several days later to pay his respects.[7]

On 2 May, 1998, Harry, Ron and Hermione returned to Hogwarts as Voldemort approached with his army. A meeting was held in the Great Hall, and when Professor McGonagall laid out the plans to evacuate the younger students from the school, Ernie indicated his willingness to fight against the Death Eaters by directly asking McGonagall about those who wished to remain behind. His question was applauded by a number of students who held similar feelings.

Although others perceived Ernie as pompous, he was loyal to his friends and always honest. He was never afraid to speak his mind, and could readily admit when he was wrong. As a student, he was hard-working and strived to get good marks, which was particularly evident in his fifth year, when he claimed to be revising for his O.W.Ls around nine hours a day.[3][6] He also displayed great bravery and loyalty in his participation in the Battle of Hogwarts[8].

Ernie did however seem to have a habit of jumping to conclusions regarding Harry Potter; in his second year he believed and ultimately spread a rumour that Harry was the Heir of Slytherin. During his Fourth year, Ernie made his disapproval of Harry's participation in the Triwizard Tournament as an illegal fourth champion clear, claiming Harry had done it to steal the spotlight from Hufflepuff.[5]

Despite being a pure-blood and proud of it, Ernie never displayed any prejudice towards those of different blood status as many other pure-bloods did. In fact, his two closest friends at Hogwarts were a Muggle-born, Justin Finch-Fletchley, and a half-blood, Hannah Abbott, and he looked out for both of them throughout his studies. Ernie was also one of the first to openly express his support of Harry and Albus Dumbledore during the Ministry's smear campaign against them.

Ernie and Hannah Abbott were close friends during their time at Hogwarts, they both had been sorted into the house of Hufflepuff. They discussed the possibility of Harry being the Heir of Slytherin together during their second year, and Hannah appeared easily influenced by Ernie's opinion.[3] Both Hannah and Ernie shared the hobby of collecting Chocolate Frog Cards. They both also strongly supported Cedric Diggory as the School Champion during the Triwizard Tournament,[5] and in 1995 they both became the Hufflepuff Prefects in their year[6].

Ernie's other closest friend was Justin Finch-Fletchley. During the Duelling Club in 1992, he moved to protect his friend when it seemed that Harry Potter was encouraging a snake to attack him. Believing Harry to be the Heir of Slytherin, and that Justin was his next target, he told Justin to hide in the dormitories. When Justin later turned up petrified, Ernie was very upset and angry with Harry until convinced that he was not the culprit behind the attack on his friend.[3]

During the Chamber of Secrets affair, Ernie held the suspicion that Harry Potter was the Heir of Slytherin when he saw him speak Parseltongue to a snake during the Duelling Club, and believed he was encouraging it to attack Justin Finch-Fletchley. His suspicions were raised when Justin became the next victim of "Slytherin's Monster". However, when Hermione Granger suffered the same fate, Ernie realised that it could not be Harry since he would never harm one of his best friends. To make amends, Ernie gave Harry a public apology.[3]

Ernie is a short form of the name Ernest, which is derived from the German word ernst, meaning "honest" or "trustworthy", or eornost, meaning "serious"[9]. It is the name of the main character in Oscar Wilde's The Importance of Being Earnest. Ernie could also be named after J. K. Rowling's paternal grandfather, Ernie Rowling.